Various additives for improving release from the mold are customary in the silicone rubber industry. The additives are either sprayed on the surface of the metal molds as external mold release agents, or added as internal mold release agents to the rubber to be processed.
Aqueous solutions of soaps are often used as external mold release agents for polysiloxane rubber compositions which vulcanize to elastomers under the influence of heat. They have the disadvantage that in the course of time they form a deposit on the mold surface, which leads to surface defects on the shaped object, therefore they have to be cleaned often.
Fatty acids or metal salts thereof are used as internal mold release agents for polysiloxane rubber compositions which vulcanize to elastomers under the influence of heat. However, mechanical properties in the polysiloxane rubber compositions are impaired as a result. In particular, the compression set, which represents the restoring force of the vulcanizate after exposure to heat, is increased. A low compression set is important for sealing components. Furthermore, fatty acids, and especially metal salts thereof, cause discoloration of the vulcanizate. The metal salts of the fatty acids are insoluble in the polysiloxane rubber compositions and therefore reduce their transparency.
EP-A-651 020 describes the use of fatty acids or metal salts thereof in combination with water as internal mold release agents. Although the water improves the mold release properties, it leads to blisters in the rubber during vulcanization. In addition, such mixtures lose the water as storage time lengthens.